The invention relates to an electro-pneumatic signal converter, having a piezo-electric bending element which is disposed in a sealed transmitter casing from which a signal output issues, and which when an electrical voltage is applied bends to thus control a pneumatic signal transmitter which consists of an air inlet seating and an air outlet seating provided opposite each other in the transmitter casing, the piezo-electric bending element being pretensioned against the air inlet seating by a spring.
A signal converter of this construction is known from DE-OS No. 3400645. In this, the piezo-electric bending element is clamped along an edge region in the transmitter casing and is elastically urged against the air inlet seating. As soon as an electrical voltage is applied to the piezo-electric bending element, this latter lifts away from the air inlet seating and closes the air outlet seating. The signal output, which was previously vented through the air outlet seating, is connected by this means to the air inlet seating, so that the pressure medium fed through the air inlet seating emerges at the signal output as a pneumatic pressure signal. As soon as the voltage at the piezo-electric bending element is switched off or reversed in polarity, the bending element returns to its initial position, so that the signal output is again vented. This electro-pneumatic signal converter is characterised by a small energy requirement. It operates without significant energy comsumption, so that it is able to advantageously replace the conventional solenoid valves for the electrical operation of pneumatic circuits and apparatus, e.g. for the servo control of valves.
The object of the invention is to improve this signal converter, and in particular to simplify its manufacture, to provide more accurate guiding of the piezo-electric bending element during its control movement, and to reduce the response time of the pneumatic part.
This object is attained according to the invention in that the piezo-electric bending element is disposed in a chamber which closely surrounds it in the transmitter casing, support zones spaced-apart in the axial direction of the bending element being provided for this latter, and in that the piezo-electric bending element is retained and urged against the support zones by a guide spring fixed in the transmitter casing. By virtue of the substantial reduction in the volume of the chamber in the sealed transmitter casing, short response times are also attained for small pneumatic throughputs through the signal converter, so that for example in the servo control of pneumatic valves, short valve switching times can be attained. The narrow chamber in which the mobile bending element is disposed requires the bending element to be precisely fixed and guided, and this is attained by the arrangement of support zones according to the invention, and in particular by the guide spring provided by the invention. Overall, by this means a precise guiding of the piezo-electric bending element is attained, making advantageous operation of the signal converter possible. Moreover, in spite of its constructional and operational accuracy, the arrangement according to the invention is characterised by a surprising simplicity.
In a preferred embodiment of the invention, the piezo-electric bending element is supported on the transmitter casing on a support bearing and, axially spaced apart therefrom, on a rocking fulcrum consisting of punctiform or linear rocker bearings disposed on both sides of the longitudinal axis of the piezo-electric bending element. In this case the support zones are disposed on the transmitter casing itself, and therefore exactly defined in terms of their spatial position. The support bearing can be in the form of an electrical contact pin which is inserted into the transmitter casing and simultaneously serves for feeding the voltage.
According to an advantageous embodiment of the invention, the guide spring which urges the piezo-electric bending element against the support zones can act on the piezo-electric bending element in a region lying in an axial direction between the support bearing and the rocking fulcrum by way of at least one preferably punctiform or linear contact zone, e.g. by way of a spherical bead. By this means a defined point of action of the spring force is attained. Preferably, the contact zone or contact zones of the guide spring are provided on an axially extending tongue thereof, which is separated from the edge regions by cut-out portions. This construction prevents the point of action of the spring force becoming displaced should the guide spring become distorted. For fixing the guide spring in the transmitter casing, the guide spring can be provided with fixing lugs projecting laterally beyond the piezo-electric bending element, and to which pins retained in the transmitter casing are fixed, the pins preferably being in the form of contact pins which simultaneously serve for the voltage feed. By means of this construction, the guide spring is only clamped between two casing halves, the pins providing exact centering. Feeding the electrical voltage to the piezo-electric bending element advantageously by way of the guide spring results in simple construction, if the pins provided for centering also act as contact pins.
In the signal converter according to the invention, in addition to fixing the guide spring in the casing it is also necessary to make a firm connection between the guide spring and the piezo-electric bending element. For this purpose the guide spring can comprise retention lugs which laterally project beyond the piezo-electric bending element, to be bent around the edges of the piezo-electric bending element and be fixed to this latter, preferably by firm cementing.
As only a limited space is available in the transmitter casing for the guide spring, this latter is advantageously in the form of a flat cantilever spring. It can be bent upwards against the support zones and thus provide the required spring force. According to a further embodiment of the invention, the guide spring comprises a tongue which is bent away from the piezo-electric bending element out of the plane of the guide spring and has its end bent towards the bending element so that the end of the tongue lies against the piezo-electric bending element. The end of the tongue bent in this manner can be bevelled, so that the edge which lies against the piezo-electric bending element forms an exactly defined zone of action of the spring force.
In the signal converter according to the invention, the transmitter casing must obviously be so constructed that it sufficiently accurately and, in relation to the air inlet seating and air outlet seating, sufficiently rigidly supports the guide spring which retains and guides the piezo-electric bending element. In order to attain this, the transmitter casing consists of solid plates of a rigid material, in which flat recesses are provided for the piezo-electric bending element and its guide spring. The recesses in the plates of the transmitter casing closely surround the piezo-electric bending element and the associated guide spring, leaving free only the space necessary for the movement and that resulting from the overdimensioning required by manufacturing tolerances. This construction ensures the required small inherent volume of the signal converter, without the movements of the piezo-electric bending element and its guide spring being hindered by distortion or suchlike of the casing parts.
In order to simplify the voltage feed to the piezo-electric bending element, the transmitter casing can consist of an electrically insulating material, preferably ceramic or glass. Both materials are sufficiently firm to ensure the required flexural stiffness and resistance to distortion. The plates forming the transmitter casing lie on each other in an air-tight manner about the recesses, and are preferably cemented together.
Further details and advantages of the invention will be apparent from the description of embodiments given hereinafter with reference to the drawings.